1. Field of the Invention
The present invention relates to ring laser gyroscopes.
2. Discussion of Prior Art
Ring laser gyroscopes have one or more sensitive axes and for each axis there is provided a gas-filled cavity disposed in a plane at right angles to the associated axis. Each cavity comprises a plurality of reflective surfaces, usually discrete mirrors, defining a polygonal path for two laser beams travelling in opposite directions in a closed loop around the cavity. The laser beams are regeneratively amplified at frequencies for which the path length equals an integral number of wave lengths.
A triaxial ring laser gyroscope is disclosed in UK Patent Number 2076213B which comprises three mutually orthogonal square cavities. The cavities are interconnected at the corners so that they share mirrors thus reducing the number of mirrors required to six. The cavities form a regular octahedron and are machined from a unitary block of dielectric material such as Zeredur. The starting geometry of the block is a cube which has at the centres of its faces the six vertices of the octahedron. The vertices are the locations of the six mirrors and the faces of the cube define the mirror mounting planes.
The cube is particularly convenient to manufacture to high accuracy and greatly facilitates the precision machining of the laser bores.
Large bores are ground from the octahedron vertices toward the centre of the block to form gas reservoirs. These reservoirs extend laser life by reducing the effect of the finite gas permeability of the block and electrodes.
Laser gyroscopes are conventionally excited by direct current applied between metal electrodes sealed into the cavities containing the laser gain medium, often a low pressure helium-neon gas mixture, to cause gas discharge and to initiate lasing. In the triaxial ring laser gyroscope disclosed in UK Patent Number 2076213B, the corners of the cube are machined off to form mounting faces for a cathode, dither spring and so on. For each of the three cavities, two additional bores are connected to the lasing path from the exterior of the block to form anode connections. The anodes have to be symmetrically positioned with respect to the cathode end a means has to be found to prevent anode to anode discharges.
In a ring laser gyroscope formed from a unitary block of dielectric material, gas-tight seals are required at each of the mirrors, at each electrode and at a gas-fill aperture.
Known ring laser gyroscopes of the type described above have several disadvantages.
The gas-tight seals are expensive to make properly, may be prone to leakage and may be a source of failure. The cathode structure has to be large if sputtering is not significantly to degrade the life of the laser. Further, the internal surface of the cathode needs to be specially processed which is an inconvenience and cost in manufacture.
Moreover, the discharge has to be split into two counter current paths which are carefully balanced if gas flow is not to give unacceptably large gyro drift errors. This requires careful design and precision machining of the cavity bores together with use of very precise, high voltage regulating electronics. Both are expensive and may leave significant residual gas flow errors.
Furthermore, it may be difficult to ensure that the discharge initiates in the correct gas path. This requirement places restrictions on the bore geometry which further reduces the available gain. In the case of a triaxial ring laser gyroscope formed from a unitary block and in which the cavities are interconnected, the problems are especially taxing. The difficulty is in establishing an electric field of sufficient strength to initiate discharge along the required gain bores without channelling plasma into bores where it is not required but nonetheless can sustain a discharge path.
Large value ballast resistors can be used in the laser current path to suppress plasma oscillations. However, the use of these mean that the laser gyroscope has to be run on even higher voltages. The resistors waste power and, because they must be in very close physical proximity to the block in order to be effective, they are a source of thermally driven gyro error.